The invention relates to a method and apparatus for friction stir welding. More particularly, in accordance with the invention, excess heat produced in the friction stir welding process is removed so that a smoother weld surface is produced.
Friction stir welding (FSW) is a relatively new welding process for joining together parts of materials such as metals, plastics, and other materials that will soften and commingle under applied frictional heat to become integrally connected. A detailed description of the FSW apparatus and process may be found in Patent Publications WO 93/10935; WO 95/26254; and U.S. Pat. No. 5,460,317, all of which are hereby fully incorporated by reference. One of the useful apparatus for FSW is shown in FIGS. 1A and 1B. As shown, two parts, exemplified by plates 10Axe2x80x2, and 10Bxe2x80x2 are aligned so that edges of the plates to be welded together are held in direct contact on a backing plate 12xe2x80x2. An FSW tool W has a shoulder 14xe2x80x2 at its distal end, and a non-consumable welding pin 16xe2x80x2 extending downwards centrally from the shoulder. As the rotating tool Wxe2x80x2 is brought into contact with the interface between plates 10Bxe2x80x2 and 10Axe2x80x2, the rotating pin 16xe2x80x2 is forced into contact with the material of both plates, as shown. The rotation of the pin in the material and rubbing of the shoulder against the upper surface of the material produces a large amount of frictional heating of both the welding tool and the plate interface. This heat softens the material of the plates in the vicinity of the rotating pin and shoulder, causing commingling of material, which upon hardening, forms a weld. The tool is moved longitudinally along the interface between plates 10Axe2x80x2 and 10Bxe2x80x2, thereby forming an elongate weld all along the interface between the plates. The welding tool""s shoulder 14xe2x80x2 prevents softened material from the plates from escaping upwards, and forces the material into the weld joint. When the weld is completed, the welding tool is retracted.
Welds produced by the prior art friction stir welding process can produce smooth welds for certain materials, but for non-extrudable aluminum alloys, the maximum spindle speed is severely limited by adherence of the material to the welding tool shoulder and pin. For these alloys, exemplified by aluminum alloys 7075, 2014, 2090, and 2024, as the spindle speed increases, and correspondingly the heat input to the weld increases, the surface texture of the upper surface of the weld degrades by becoming rougher. At higher spindle speeds, and higher heat input, the aluminum material adheres and builds up on the welding tool shoulder, tearing away material from the sides of the weld surface. For long welds, this condition can cause such excessive buildup that continuing the weld becomes impossible. Also, the overheated welding tool can sometimes partially tear away surface material from the center of the weld surface, producing a xe2x80x9cfish scalexe2x80x9d appearance on the upper surface of the weld which progressively worsens along the length of the weld. For certain applications such a rough weld surface is undesirable, and requires additional machining to produce a smooth surface. Rough surfaces often provide points of initiation of fatigue cracks, and are therefore generally undesirable, especially if the welded part is to be used under conditions that could cause fatigue, such as cyclical conditions of applied load. There exists a need for a FSW process that produces a weld of reduced surface roughness that would not require subsequent machining, for most applications, and that would have a uniform, smooth surface texture.
The invention provides a method and apparatus for producing a friction stir weld of difficult to weld materials, such as non-extrudable aluminum alloys, that has a smoother surface than heretofore achieved with conventional friction stir welding equipment. The weld is produced at higher speeds and has a commercially acceptable surface smoothness so that it does not require subsequent machining for most purposes.
In accordance with the invention, it has now been found that the rate of welding limitation on non-extrudable materials, imposed by the increasing roughness of the weld surface as welding rate increases, is caused by excessive heat generated during the friction stir welding process at the surfaces of contact between the tool and the workpiece being welded. While a certain amount of heat is necessary to cause softening of the material to form the weld, excessive heat causes the softened material to adhere to the rotating pin and shoulder of the friction stir welding tool. The rotational and lateral movement of the tool against these adhesive-type forces causes the irregular weld surface. Therefore, the invention provides a method of friction stir welding that includes the step of simultaneously cooling the welding tool during the welding process to remove excess heat. This method allows a significant increase in welding rate, preferably at least about a 20% increase, and more preferably at least about a 100% increase, while maintaining an acceptable weld smoothness. Moreover, the invention provides apparatus for friction stir welding that are cooled by a coolant.
In one embodiment the coolant is circulated in the body of the tool to remove excess heat. In this embodiment, the friction stir welding tool of the invention has a tool body with a rotatable, usually non-consumable, pin and shoulder at its distal end that are adapted for stir welding parts together. The tool body has an internal space that is in heat-conducting communication with the pin, and preferably also the shoulder, of the welding tool. The internal space is adapted for flowing a coolant therethrough to remove excess heat from the tool, including heat conducted from the shoulder and pin.
In another embodiment, heat is removed from the friction stir welding tool by a jacket that surrounds a distal portion of the tool body. The jacket has an inlet that is in fluid communication with a source of coolant, and an outlet for exit of heated coolant. Thus, when the tool is in use, coolant flows through the jacket removing heat from the tool, so that excess heat is removed from the rotatable pin and shoulder.
In another embodiment, the removal of heat is achieved by spraying a coolant (such as cold air, or a liquid coolant, such as water) onto the tool, and surrounding surfaces being welded, during the welding step. Preferably, the tool portion being cooled is equipped with fins to facilitate heat removal.
In accordance with the invention, friction stir welds of even non-extrudable aluminum alloys are produced at commercially useful rates and have such a reduced surface roughness texture that they may be used commercially.